Interesting you should ask for more verification of my claim to fewer bugs because I'm about to embark on using Haskell in the new startup I'm working for and they may require stronger reasoning than I have been providing.
I've been thinking about how to qualify the claim logically or maybe quantify it. Anywho, aside from that I can tell you that the vast majority of bugs I introduce into my Python code (or other languages) are caught by GHC - these are bugs that purely have to do with my inability to remember what something is or is doing, or fear of refactoring something, or some tangled mess of types that are harder to reason about in Python because they're in my head instead of encoded in Haskell.
I've discovered that when I've been able to compile my Haskell programs the bugs I find in them are usually business logic bugs now. Occasionally I'll make use of QuickCheck / HUnit for automated property testing and unit testing, and that will help catch those business logic bugs.
Very subjective and anecdotal but I can promise you that the experience is very real.
> In isolation, that's a bit hand wavy IMO.
Sure, but if you take into account Haskell's denotational semantics vs. other language's operational semantics it should be pretty clear that Haskell inherits Mathematic's idiom for succinct expression. That idiom also informed the language designers when building Haskell's grammar and syntax, it's very flexible and abstract - both in the essence of the language (it's semantics) and its modality.
When reading idiomatic Haskell, there's a lot of information packed into a line, conceptually and syntactically.
> This is something I hear quite often from the Haskell community...
Sure, it probably borders on it, but in many ways other language communities commit the bandwagon fallacy (Go is one) - so I think you can poke holes all over the place. The fact though that proof proving systems and dependently typed languages like Agda and Idris (which are the future and I think will supplant Haskell unless Haskell can evolve) are being written in Haskell which speaks to the level of intellect and forward thinking in the community.